The idea is that identifying people infected with HIV and getting them in effective treatment as soon as possible not only prevents them from getting sick but almost eliminates the risk they'll pass the virus on to others.

Last summer a big study showed that people with HIV are 96 percent less likely to pass the virus on if they faithfully take antiviral medicine. Momentum behind treatment-as-prevention has grown since then.

This is a big change. For a long time in the world of HIV and AIDS, the conventional wisdom has been to delay treatment until people show signs of damage to their immune system. Partly this is because the drugs have side effects (although some are now easier to take), and partly because few people thought medical treatment itself could slow the spread of HIV.

"After many years of frustration, it is a transformational moment in the course of this epidemic," says Kevin Cranston, head of infectious disease control at the Massachusetts Department of Public Health. "Universal treatment can in fact result in an epidemic that looks to be petering out."

Massachusetts is one of the places where it's actually beginning to happen. There are so many people on treatment that new infections are dwindling — a tipping point after years of increases.

"We, unlike most states and frankly most cities in the country, are seeing a sharp decline in the number of new cases of HIV over the past decade," Cranston says. New infections have fallen by nearly half and deaths from AIDS are at an all-time low in Massachusetts.

"We're approaching a level where the rate of transmission does not sustain an epidemic," he says.

Preventing HIV infection requires not only treating almost everybody with the virus but getting them to take antiviral pills faithfully, so that blood levels of the virus are suppressed to levels undetectable with standard tests. In Massachusetts, indications are that more than 70 percent of people on treatment have undetectable virus — compared with a national rate of under 30 percent.

But even in Massachusetts, many people still don't know they have HIV or they wait too long to get tested. Nearly 1 in 3 new cases already has full-blown AIDS or is close to it.

"It's a tragedy if someone doesn't present for care until they have an AIDS diagnosis," Cranston says. It means they've been infected for years, and able to infect others.

It's true all over the country. Nearly a quarter-million Americans have HIV without knowing it. And many others who do know they're infected are not getting treated.

That's why there's a big push now to do what AIDS experts are calling "test and treat" — reach out to people who might be infected, get them tested, and if they're HIV-positive get them into treatment as soon as possible.

But AIDS researcher Rochelle Walensky at the Massachusetts General Hospital cautions against expecting too much. She says test-and-treat programs are never going to get 100 percent of people into effective, long-term treatment. But they should get as close as they can.

"If we could find everybody with HIV, link them to care, make sure they were getting treatment and doing well, we could make a huge dent in this epidemic," Walensky says. "Could we eradicate it? Perhaps ... but it'll take a long time."

The problem is, many of those not yet in treatment have other, more urgent priorities than getting tested for HIV — and then taking pills for it without fail for the rest of their lives.

Kenya Jackson is one of those HIV patients who have been very hard to keep in treatment. He's a 52-year-old Boston man with lots of problems. On top of his HIV infection, Jackson has kidney failure, hepatitis B and high blood pressure. He's also been homeless for years.

Jackson found out he had the virus way back in 1999. He thought he was going to die.

"That rocked my world from then on," he says, "'because when I was growing up hearing about this disease, a lot of people weren't making it."

While Jackson is on his thrice-weekly dialysis treatment, a community health worker comes by to give him his medication. He swallows 10 pills; three of them keep HIV in check.

For most of the past 13 years, however, Jackson hasn't taken his HIV medication every day. "When I was diagnosed with the virus, I was kind of leery about taking the pills," he says. "They kept bothering my stomach. So I kept telling them I'm taking it, but I wasn't."

Eight months ago he got into a program called PACT, or Prevention and Access to Care and Treatment. PACT arranged for a different regimen that doesn't make Jackson sick. A community health worker named Greg Jules makes sure Jackson takes his medicine and otherwise takes care of himself.

"It's been challenging, because Kenya has a long history of the illness. He has a lot of ups and downs," Jules says. "My whole job is to try to make him see I'm in his corner, I'm looking out for him. And whatever we have to do to get there, I'm willing to do."

That means trying to make Jackson's life less chaotic, more predictable. So far, it's working. When Jackson's dialysis session is over, Jules takes him to sign the lease on an apartment. He won't be homeless anymore.

"I'm very happy for you, very excited," Jules tells Jackson. "You're going to have your own place. So therefore, we don't want to do anything to impact that. You want to go into a new way now."

Jackson promises to be "a new Kenya" and stay on his HIV medication this time — to keep himself well, and to avoid infecting anyone else.

In his case, as in many others, it takes a lot of one-on-one attention to turn HIV prevention into HIV treatment. But veterans of the fight against AIDS say it's going to be necessary to break the cycle of infection.